1. Field of the Invention
This invention relates generally to magnetic heads in disk drives, and more particularly to magnetic write heads with bilayer pole tips and methods of making the same.
2. Description of the Related Art
A write head is typically combined with a magnetoresistive (MR) or giant magnetoresistive (GMR) read head to form a merged head, certain elements of which are exposed at an air bearing surface (ABS). The write head comprises first and second pole pieces connected at a back gap that is recessed from the ABS. The first and second pole pieces terminate at the ABS where they define first and second pole tips, respectively. An insulation stack, which comprises a plurality of insulation layers, is sandwiched between the first and second pole pieces, and a coil layer is embedded in the insulation stack. A processing circuit is connected to the coil layer for conducting write current through the coil layer which, in turn, induces magnetic write fields in the first and second pole pieces. A non-magnetic gap layer is sandwiched between the first and second pole tips. Write fields of the first and second pole tips at the ABS fringe across the gap layer. In a magnetic disk drive, a magnetic disk is rotated adjacent to, and a short distance (fly height) from, the ABS so that the write fields magnetize the disk along circular tracks. The written circular tracks then contain information in the form of magnetized segments with fields detectable by the MR or GMR read head.
An MR read head includes an MR sensor sandwiched between first and second non-magnetic gap layers, and located at the ABS. The first and second gap layers and the MR sensor are sandwiched between first and second shield layers. In a merged MR head, the second shield layer and the first pole piece are a common layer. The MR sensor detects magnetic fields from the circular tracks of the rotating disk by a change in resistance that corresponds to the strength of the fields. A sense current is conducted through the MR sensor, where changes in resistance cause voltage changes that are received by the processing circuitry as readback signals.
A GMR read head includes a GMR sensor which manifests the GMR effect. In the GMR sensor, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g., nickel-iron, cobalt, or nickel-iron-cobalt) separated by a layer of nonmagnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. A GMR head is typically associated with a design in which the second shield layer and first pole piece are not a common layer. These pieces are separated by a non-magnetic material, such as alumina, or a metal that can be deposited using a physical vapor deposition technique or an electro-plating technique, for example.
One or more heads may be employed in a magnetic disk drive for reading and writing information on circular tracks of a rotating disk. A merged head is mounted on a slider that is carried on a suspension. The suspension is mounted to an actuator which rotates the magnetic head to locations corresponding to desired tracks. As the disk rotates, an air layer (an “air bearing”) is generated between the rotating disk and an air bearing surface (ABS) of the slider. A force of the air bearing against the air bearing surface is opposed by an opposite loading force of the suspension, causing the magnetic head to be suspended a slight distance (flying height) from the surface of the disk. Flying heights are typically less than 0.02 μm in today's disk drives.
Consumer demand for disk drives with larger storage capacity and higher data areal density requires, in part, improvement of the performance of the write head. There are several conventional techniques used for making an improved write head. For example, one technique involves electroplating a pedestal over the first pole piece as part of the first pole tip and then notching the pedestal. The use of the notched pedestal is advantageous since it generally reduces the head's fringing field, which is the field that extends over the adjacent track when the current track is being written to. The reason that the notched pedestal reduces the fringing field is because the field is captured by the notched pedestal instead of being spread out laterally over the flat portion of the first pole piece on each side of the second pole tip.
Another conventional technique involves notching the pole tip of the first pole piece. Here, the second pole and its pole tip are first frame-plated on top of the gap layer. After depositing a seed layer on the gap layer, a photoresist layer is spun on the seed layer, imaged with light, and developed to provide an opening surrounded by a resist wall for plating the second pole piece and second pole tip. Once the second pole tip is formed, the first pole piece is notched opposite the first and second bottom corners of the second pole tip. A prior art process for notching the first pole piece entails ion beam milling the gap layer and the first pole piece, employing the second pole tip as a mask. According to this prior art process (typified in U.S. Pat. No. 5,452,164 and U.S. Pat. No. 5,438,747), the gap layer is typically alumina and the first and second pole pieces and pole tips are typically Permalloy (NiFe). Notching improves the transfer of flux between the second pole tip and the first pole piece, as the flux will transfer to the pedestal portion of the first pole piece instead of its larger expanse.
Write heads must continuously be improved to provide for better overwrite (OW) capabilities and reduced fringing fields. One prior art technique described in U.S. Pat. No. 5,864,450A1 teaches the utilization of an additional material on top of the pole tip which has a higher saturation magnetization than that of the material beneath it. This improves the write performance of the write head. However, this technique is limited in application to a write head that does not have a pedestal on its pole piece and no fixed throat height. Both materials have a relatively low magnetic moment by today's standards.
What is needed is an improved write head which provides for superior writing capabilities, including better overwrite capabilities and reduced fringing fields. Better methods for making such magnetic heads are also needed.